A constant current driver circuit for two LED loads (Chinese patent application No. 200910155848.0) is shown in FIG. 1, in which a capacitor Cb can maintain the currents of two LED loads (A1 and A2) to be equal. In FIG. 1 the constant current driver circuit for only two LED loads is shown. A constant current driver circuit for more than two LED loads can be implemented as the circuits shown in FIG. 2 and FIG. 3, in which current sharing among multiple LED loads is achieved by a current sharing transformer T1. The circuit in FIG. 2 is applicable to drive even number of LED loads, and the circuit in FIG. 3 is applicable to drive odd number of LED loads.
In the circuit shown in FIG. 1, if one of the LED loads is opened and the other path of the LED loads is ensured to operate normally, an abnormal overvoltage will occur at the output of the opened load, such that the driver circuit will be damaged. Therefore, an additional open circuit protection circuit is needed.
An LED driver circuit with an open circuit protection circuit in the prior art is shown in FIG. 4.
Each of the LED loads is connected in parallel to an open circuit protection circuit K. When an abnormal overvoltage is detected at the output, a thyristor (SCR1, SCR2) is turned on and a load current flows through the thyristor, so as to avoid the abnormal output voltage due to the open circuit of the load.
However, the open circuit protection circuit K has the following disadvantages: when the output voltage is abnormal, the thyristor is turned on, and short-circuit discharge of an output filter capacitor (Co3, Co4) will cause a large impulse current, such that current stress in the circuit is increased, the cost of the circuit is increased and the reliability of the circuit is reduced.